Ground fault detection for power distribution system



Jan. 27, 1970 R. s. THURSTON GROUND FAULT DETECTION FOR POWERDISTRIBUTION SYSTEM Filed Jan. 10, 1967 lllillllllllllllllllllu mm Q 3a? r 8 m m 7 A i u l/ n E 9 w m EW 7 B F G mum 2 R 8 M& o m {7 s v UR 74 6 4V6 8 ll L j 1 wk E E I l||l|||||||lLQ M u H O 6 f B m u 3 3 r,

A m m w M I WIIL Q N m N ARCING FAULT I INVENTOR. ROBERT S. THURSTONLOAD ATTORNEYS United States Patent 3,492,533 GROUND FAULT DETECTION FORPOWER DISTRIBUTION SYSTEM Robert S. Thurston, Needham, Mass., assignor,by mesne assignments, to The Kelek Company, Norwood, Mass.,

a corporation of Connecticut Filed Jan. 10, 1967, Ser. No. 608,429 Int.Cl. H02h 3/34, 1/02 US. Cl. 31718 9 Claims ABSTRACT OF THE DISCLOSURE Aground fault detector for a three-phase, four wire distribution unit isadapted to open a switch upon the occurrence of an inequality in thevoltages between the phase lines and the conductive shield for thefeeder lines. The latter is connected with the neutral wire from thepower source through an impedance that completes a circuit path forfault current and also limits such current.-

The present invention relates generally to means for detecting thepresence of a shorting or arcing fault between a conductor in amulti-phase feeder line and its grounded metal conduit or enclosure.More particularly, it relates to means for detecting such a fault by thepresence of an inequality in the voltages between the phase lines andthe metal enclosure of the switchboard or distribution equipment, suchmeans being useful, for example, to trip a circuit breaker or to open aswitch by actuation of a switch motor or pneumatic switch operator.

In a typical three-phase, four wire power distribution system there area grounded neutral line and a plurality of phase lines at equal voltagesabove the neutral line. These lines extend from a power source which maytake the form of a distribution transformer, a transformer bank or apower company network, connected with the power generating equipment.The phase and neutral lines are connected to a switchboard whichincludes one or more switches or circuit breakers mounted within agrounded metal enclosure. A load circuit is connected to the switchboardby a feeder cable comprising conductors in metal conduit or metalenclosed bus duct, such metal parts being connected with the switchboardenclosure. In larger systems there may be a main switch or circuitbreaker having a main bus on its load side connected with a plurality offeeder switches or circuit breakers for a plurality of cor respondingfeeder cables. The feeder cables may be of various lengths extending upto several hundreds of feet from the switchboard enclosure.

Commonly, the neutral conductor of the power transmission line, which isgrounded at the power source, is connected by a ground strap to thegrounded enclosure of the switchboard. If an arcing fault occurs betweenone of the feeder lines and its metal shield or conduit, there areparallel paths for the return current. One is along the conduit to aground connection and thence through ground to the neutral connection atthe power source. Another path is along the conduit to the enclosure ofthe switchboard, and thence through the ground strap and the neutralline back to the neutral connection of the power source. The latter pathactually carries nearly all of the fault current, and this is explainedby the fact that this path has a lower impedance than a more remote paththrough ground.

With the foregoing connections, reliance is conventionally placed uponprotective devices such as fuses or circuit breakers in the feederlines. Thus an arcing fault of sutlicient magnitude may increase thecurrent in one phase and trip its circuit breaker or blow its fuse.However, it has been found that many faults do not produce sufficientcurrent to operate such safety devices. This re- ICC sults in some casesfrom the fact that the fault occurs at a substantial distance along thefeeder cable from the switchboard, and the impedance of the feeder linelimits the fault current to a valve insufficient to trip the breaker orblow the fuse. Another disadvantage is that even though a fault currentmight be of sufficient magnitude eventually to blow the fuse or trip thebreaker, the delay in time before this occurs might allow severe damageto occur to electrical equipment of the building structure or contents.

For example, fault currents ranging from 8,500 to l1,- 000 amperesreturning through normally installed conduit have produced showers ofsparks at points along the fault return circuit such as joints where theconduit is connected in such manner to introduce resistance, for examplewhere there is corrosion, or loose lock nuts or other threads nottightened. These showers of sparks can readily ignite combustiblematerial. An arcing fault of 2,000 amperes at 480 volts can burn holesin a steel switch enclosure in two to four seconds, and can completelydestroy it in ten seconds. Tests show that a two-inch steel conduit Willburn through in about three seconds with a current of 500 amperes, andin five seconds with a current of 350 amperes. Aluminum conduit willburn through in less time.

Prior efforts to overcome these disadvantages have dealt with means forreducing the impedance of the previously described fault current returncircuit including limiting the lengths of feeder circuits to permitsufficient current flow to operate the normal short circuit protectivedevices quickly. This method often allows very large fault currents toflow until the protective devices are operated, with the attendanthazard of sparks and damage.

More recently, there have been proposals for detecting ground faults bythe presence of current in the ground strap, as illustrated by thepatent to Soares No. 3,005,932. In a three-phase distribution systemwith equally loaded phases there would normally be no current flowing inthe neutral line. According to said patent, an arcing fault in one phaseline would produce a current in the ground strap which through inductivecoupling could be used to operate a safety device. However, any sucharrangement based on fault current sensing is subject to the samecriticism previously mentioned, namely, that the fault currents may bevery substantial because of the low impedance of the return circuit, andthere may be a momentary shower of sparks or severe burning at the pointof arcing or elsewhere along the feeder cable.

The objects of this invention include providing means for detecting thepresence of an arcing fault without excessively high ground faultcurrent, thereby lessening the hazards of sparks and damage toequipment.

Having in view the foregoing and other objects hereinafter more clearlyappearing, the features of this invention reside in an arrangementwhereby the metal enclosure of the switchboard is allowed to shiftmomentarily in potential with respect to ground as the result of anarcing fault in a feeder line, and this momentary shift produces aninequality in the voltages between each of the feeder lines and theenclosure, this inequality being detected and utilized as a signal foroperation of indicating, signalling or protective equipment. A. relatedfeature is the employment of an impedance through which the groundedneutral line from the power source is connected to the enclosure of theswitchboard.

Other features include the adaptation of this novel method to permit itsuse in conjunction with other features herein described.

Still other features of the invention reside in certain details ofconstruction and in arrangements of the elements and modes of operationwhich will become evident from the following description of a preferredembodi- 3 ment, having reference to the appended drawings in which FIG.1 is a schematic circuit diagram illustrating a preferred embodiment ofthe invention; and

FIG. 2 is a vector diagram illustrating the operation of the circuit.

Referring to the drawings, at 12 there is shown a typical powerdistribution transformer for a three-phase, four-wire system, havingY-connected primary windings 14 and Y-connected secondary windings 16with a grounded neutral at a point 0. The primary windings may bedelta-connected, if desired. The secondary windings are connected withphase lines A, B and C, and a neutral line N is terminated at theneutral secondary connection O.

The transformer 12 is connected with a switchboard 26 having anenclosure 27 grounded at 28, the lines N, A, B and C taking the form ofcables or bus duct. Within the switchboard there is a main switch 29having a motor M and fuses 30. It will be apparent from the followingdiscussion that a circuit breaker may be employed in place of the switch29. The load side of the switch is connected to a feeder cablecomprising the line N and feeder lines 32, 34 and 36, these linespassing through metal conduit 38 to a load 40 grounded at 41. The loadmay be, for example, a panel board, a motor control center or any othertype of load.

From the following discussion, it will become apparent that althoughonly one feeder cable is shown, there may be more than one feeder cable,in which case the load side of the switch 29 would be connected with amain bus which would be connected in turn to each feeder cable through aseparate feeder breaker or switch.

The foregoing connections are the same as those conventionally used inany three-phase, four-wire power distribution system. We next turn toone of the points of difference, namely, the connection of the neutralline N. As stated above, in a conventional system the wire N isconnected. by a ground strap directly to the enclosure 27. According tothe present invention the line N is connected to the enclosure throughan impedance Z. This impedance may be resistive, inductive orcapacitive, or a combination of these impedances. A function of theimpedance, as herein further described, is to limit any fault currentsthat may flow through it.

An understanding of the further functions of the impedance Z may begained from considering the effect of a typical arcing fault. A commonfault is shown at 44 and comprises an are from the feeder line 36 to theconduit 38 which is at ground potential by reason of ground connectionsat points such 28 and 41. It will be apparent that the same kind offault may occur with metal enclosed bus ducts in place of conductorcables within metal conduit. The distance from the swithboard at whichsuch a fault is assumed to occur may vary anywhere from a few feet toseveral hundreds of feet in various applications.

As previously stated, there are parallel paths by which a return circuitmay be completed between the feeder line 36 at the fault 44 and theground connection at the transformer. One path is through ground fromconnections at 41 or 28 to the ground connection at 0, but any such pathwould cause the return current to flow at some distance from the feederline 36 and its corresponding phase line C. Another path is through theconduit 38 to the enclosure 27, then through the impedance Z and theneutral line N to the connection 0 at the transformer, whereby thereturn current flows along paths that are close to the conductors 36 andC. For the reason previously stated, substantially all the fault currentreturns to the transformer along the last-mentioned path.

FIG. 2 is a vector voltage diagram illustrating the result of a faultcurrent flowing in the feeder line 36. Broken lines 0A, OB and OCrepresent the voltage vectors from the neutral connection 0 to each ofthe phase lines A, B and C, respectively. A voltage vector OD representsthe momentary difference in potential between the enclosure 27 at apoint such as D and the connection 0 due to the fault current which isflowing through the impedance Z. The vectors DA, DB and DC represent thedifferences in potential between the enclosure 27 and each of the feederlines 32, 34 and 36.

According to this invention, use is made of the resultant difference orunbalance in the magnitudes of the voltages DA, DB and DC, produced bythe ground fault, to signal the presence of such fault and to operateprotective means, illustrated in the drawing by the main switch motor M.This unbalance is detected by a voltage unbalance relay 46, outlined inthe drawing by a broken line. Relays of this type or their functionalequivalents are standard pieces of equipment now in use in otherapplications and the particular form of the relay forms no part of thepresent invention. However, for completeness of description a particularform of voltage unbalance relay is shown in the drawing and describedbelow. Further description of its circuit may be found in Patent No.3,155,880 dated Nov. 3, 1964 to Louis E. Salina.

The illustrated relay 46 includes transformers 52, 54 and 56 andtransistors T1 and T2, together with an output transformer 58 andrelated circuits. The primary windings of the transformers 52, 54 and 56are Y-connected by a wire 60 to a common connection D on the enclosure27. The secondary windings are connected to identical single phaserectifier circuits 62, with equal resistors 64, 66 and 68 across whichD.C. voltages appear with negative connections in common.

The primary windings of these transformers may be rated in some casesfor a different voltage than that represented by one of the vectors OA,OB or 00, in which case they are connected Y-Y to the secondaries ofthree intermediate transformers. The wire 60 is connected to the commonconnection of such secondaries rather than to the enclosure 27 at thepoint D. The primary windings of such intermediate transformers are thenY-connected to the phase lines A, B and C, with their common connectionmade to the enclosure 27 at the point D.

Wires 70 and 72 connected respectively to the emitter and base of thetransistor T1 are connected with resistors 64, 66 and 68 through a pairof Y-connected rectifier circuits 74 and 76.

An inspection of this circuit will show that if the voltages on theprimary windings of the transformers 52, 54 and 56 are equal, the D.C.voltages on the resistors 64, 66 and 68 will also be equal and thecircuit will be in balance with no current flowing through the wires 70and 72. However, if the voltages become unequal in magnitude, a negativebase-to-emitter bias voltage will appear on the transistor T1. Thiscondition arises when there is a difference in the lengths of thevectors DA, DB and DC in FIG. 2.

The transistors T1 is normally conductive because a positivebase-to-emitter bias is supplied by a battery B, which produces the biasacross a resistor 78. The transistor T2 is also normally conductivebecause a negative base-to-emitter bias is supplied by the battery B,which produces the bias across a resistor 79 as a result of conductionin the transistor T 1.

Oscillation of the circuit results from inductive coupling between awinding 80 in the collector circuit of the transistor T2 and a winding82 connected with the base of the transistor T1 in proper phase toproduce oscillation.

In the case of an unbalance in the voltages across the primary windingsof the transformers 52, 54 and 56, the resulting voltage across thewires 70 and 72 is of such polarity as to reverse the base-to-emitterbias on the transistor T1, causing it to cease conduction. Thisinterrupts the oscillation.

A secondary winding 84 on the transformer 58 is connected through arectifying bridge circuit 86 to the operating coil of a relay R. Thisrelay is shown in the normally energized condition with its contacts 88open, thereby interrupting the energizing circuit of the motor M. Whenthe oscillation of the transistor T2 is stopped the relay R isdeenergized, the contacts 88 are closed and the motor M is energized toopen the main switch 29.

If desired, the relay R may actuate an adjustable time delay relay todelay the operation of the motor M or other protective means.

The advantages of the above-described system largely result from thefact that it imposes an upper limit on the fault current that can flowbefore operation of the motor M or other protective device. Theimpedance Z limits the fault current to a maximum value which is notlikely to cause dangerous showers of sparks at poor joints in theconduit or bus enclosure system, and damage to conduits, enclosures andconductors. This also means that an upper limit is placed on the currentto be interrupted by the switch 29 or other protective device, whichreduces or prevents any damage to such device, such as wear on itsarcing contacts.

The above-described system is effective to operate the switch 29 orother protective device in two to three seconds after an arcing faultoccurs.

The circuit is reset by closing the switch 29 after the fault has beeneliminated.

It will be evident that although batteries are shown as power suppliesfor the motor M and the transistors, this is merely to simplify thedescription, and if desired they may be replaced by suitableconventional rectifying circuits energized by the lines A, B and C, orthe motor M may be an alternating current motor receiving its power fromthe lines A, B or C.

Other variations in the construction and arrangement of the parts canalso be accomplished in accordance with recognized practices, withoutdeparting from the spirit and scope of this invention.

Having thus described the invention, I claim:

1. Ground fault detection means for a three-phase, four wiredistribution unit, said detection means comprising the combination ofmeans to connect three feeder lines having a grounded conductive shieldto one side of the distribution unit,

means to connect three phase lines from a power source to the other sideof the distribution unit,

a current limiting impedance electrically connecting the conductiveshield with a grounded neutral line from the power source, and

a detection device having a first input connection electricallyconnected to the conductive shield, three input connections electricallyconnected to the respective phase lines, and means to produce a faultsignal upon the occurrence of an inequality in the magnitudes ofvoltages between each of said three input connections and said firstinput connection.

2. The combination according tg plain; 1 ip which the distribution unitis a switch,

3. The combination according to claim 1 in which the impedance is acurrent limiting resistor.

4. The combination according to claim 1 in which the distribution unitis a switch connecting the feeder lines with the phase lines, and thedetection device is adapted to open the switch upon detecting the saidinequality.

5. The combination according to claim 1 in which the detection devicehas for each phase line a transformer with a primary winding connectedbetween the phase line and the said first input connection and a circuitfor detecting an inequality in the magnitudes of the voltages on thesecondary windings of the transformers.

6. The combination according to claim 4 in which the neutral line isextended with the feeder lines through the conductive shield.

7. The combination according to claim 6, in which the detection deviceis adapted to open the switch, after a time delay upon detecting theinequality.

8. Ground fault detection means for a three-phase, four wiredistribution unit having an electrically conductive enclosure, saiddetection means comprising the combination of.

means to connect three feeder lines having a conductive shield to oneside of the distribution unit with the shield electrically connected tothe enclosure, means to connect three phase lines from a power source tothe other side of the distribution unit, means to support a neutral linefrom the power source in electrically insulated relation to theenclosure,

a current limiting impedance connected between the enclosure and saidneutral line, and

a detection device having a first input connection electricallyconnected to the enclosure, three input c nnections electricallyconnected to the respective phase lines, and means to produce a faultsignal upon the occurrence of an inequality in the magnitudes ofvoltages between each of said three input connections and said firstinput connection.

9. The combination according to claim 8, in which the distribution unitis a switch connecting the feeder lines with the phase lines, and thedetection device is adapted to open the switch upon detecting the saidinequality.

References Cited UNITED STATES PATENTS 3,072,s27 1/1963 Benish 317 1s3,113,245 12/1963 Hofimann 317-18 FOREIGN PATENTS 557,551 7/1926Germany.

LEE T. HIX, Primary Examiner J. D. TRAMMELL, Assistant Examiner US. Cl.X.R. 17 7

